1. Field of the Invention
The present invention relates to an optical heterodyne measuring apparatus for effecting measurement based on a phase difference and a frequency difference between a reference and a measuring beat signals.
2. Discussion of the Prior Art
There is known an optical heterodyne measuring apparatus which is adapted to measure physical quantities associated with a subject, depending upon a phase difference or a frequency difference between a reference beat beam which consists of two laser beams having mutually perpendicular polarization planes and different frequencies, and a measuring beat beam which consists of the reference beat beam which has been reflected by the subject. Such a known optical heterodyne measuring apparatus usually uses a single reference beat beam whose beat frequency is equal to a difference between the frequencies of the two laser beams that constitute the reference beat beam. Therefore, the beat frequency of the reference beat beam is a fixed value which is determined by the frequencies of the two laser beams produced by a laser device.
In such a known optical heterodyne measuring apparatus, the limitation of the beat frequency of the reference beat beam causes insufficient accuracy of measurement of the subject, or a relatively limited range of the velocity of the subject to be measured. For example, where the beat frequency is comparatively high, the apparatus may follow a comparatively high speed or velocity of the subject, but suffers from comparatively low resolution of measurement (i.e., cannot measure a quantity in sufficiently small increments). Conversely, where the beat frequency is comparatively low, the resolution of measurement is sufficient for a subject moving at a relatively low speed, but the apparatus cannot sufficiently follow a comparatively high speed or velocity of the subject, whereby the velocity or position of the slowing moving subject cannot be accurately determined.